III-V power field effect transistors

ABSTRACT

A field effect transistor configured for use in high power applications and a method for its fabrication is disclosed. The field effect transistor is formed of III-V materials and is configured to have a breakdown voltage that is advantageous for high power applications. The field effect transistor is so configured by determining the operating voltage and the desired breakdown voltage for that operating voltage. A peak electric field is then identified that is associated with the operating voltage and desired breakdown voltage. The device is then configured to exhibit the identified peak electric field at that operating voltage. The device is so configured by selecting device features that control the electrical potential in the device drift region is achieved. These features include the use of an overlapping gate or field plate in conjunction with a barrier layer overlying the device channel, or a p-type pocket formed in a region of single-crystal III-V material formed under the device channel. The overlapping gate/field plate or p-type pocket extend into the drift region of the device, controlling the electrical potential of the device in a manner that provides the desired control of the electrical potential in the drift region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/948,897, filed on Sep. 24, 2004, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention is directed to field effect transistors for use inhigh power applications. Specifically, the present invention is directedto III-V semiconductor field effect transistors that can handle largevoltages (i.e. 20 volts and larger).

BACKGROUND OF THE INVENTION

Field Effect Transistors (FETs) are ubiquitous devices used in a widearray of applications. Certain applications require FETs which canhandle large voltages. For example, RF switches in cell phones requireFETs which can sustain more than 20 volts between the source and drain.Power amplifiers in wireless base stations often require FETs which cansustain more than 50 volts between source and drain and between the gateand drain.

FETs are transistors where the resistance between the source and drain(R_(SD)) is modulated by a gate voltage. FIG. 1 shows a typical GaAs FET100. The FET illustrated in FIG. 1 has a source contact 105 a draincontact 110 and a gate contact 115. Source contact 105 is in electricalcontact with source 120 and drain contact 110 is in electrical contactwith drain 125. The FET is formed on a semi-insulating GaAs substrate145 on which is formed an undoped GaAs buffer layer 140. The device 100also has a barrier layer 135 formed between the channel 130 and thesource 120, drain 125 and gate 115.

When the gate voltage is high enough that the channel isn't fullydepleted of charge, the device is in the on-state. In this on-state,R_(SD)(on) is low and current flows from the source 120 through thedevice channel 130 to the drain 125. When the gate voltage is low enoughthat the channel is depleted of charge, the device is said to be in theoff-state. In the off-state, R_(SD)(off) is high and little or nocurrent flows through the channel 130.

FIG. 1 is a GaAs MESFET with a semiconducting barrier layer. Thesemiconducting barrier layer has a higher band-gap energy than that ofthe underlying channel and is typically used to help reduce gate leakagecurrent. The gate 115 is a Schottky barrier metal that contacts thebarrier layer 135 to provide a Schottky insulating junction between thegate and the channel. The passivation layer 150 is deposited between thegate 115 and drain 125 and between the source 120 and gate 115 to reducesurface states in these regions. Surface states lead to undesirabledevice performance along with stability and reliability problems. Atypical surface passivation layer is SiO_(2.) Silicon nitride (SiN) isanother example of a typical passivation layer.

Typical semiconductor substrates used to form power transistors aresilicon (Si) and large band-gap III-V compound semiconductors. Galliumarsenide (GaAs) and gallium nitride (GaN) are examples of the materialsreferred to as “III-V” materials herein. III-V substrates have severaldesirable material properties for use in power FETs when compared to Si.These properties include high electron mobility (which leads to improvedhigh frequency performance and lower R_(SD)(on)) and higher bandgapenergies (which lead to lower impact ionization and thus higher BV).

Si-based FETs are widely used in power amplifiers for base-stations thatrequire high output power with a corresponding drain to source breakdownvoltage (BV_(DS)(off)) that is greater than 60 volts, but have higherR_(SD) and lower maximum power gain frequency (f_(max)) compared to GaAsFETS. These silicon devices are typically Si LDMOS transistors.

GaAs FETs are typically used in applications when higher speed or lowerR_(SD)(on) is required but BV_(DS) need not be as high (e.g. when theoff-state breakdown voltage BV_(DS)(off) is about 20 volts).Metal-Semiconductor-FETs (MESFETs) andHigh-Electron-Mobility-Transistors(HEMTs) are examples of these devices.Examples of such applications are RF switches and lower power amplifiersused in mobile handsets.

However, in any RF power amplifier application it is desirable either toincrease BV_(DS) without increasing R_(SD)(on) or to reduce R_(SD)(on)without reducing BV_(DS). There are many known approaches for reducingthe peak electrostatic field of FETs for use in high power applicationsto address this objective. One approach is a device known as the RESURF(for reduced surface field) device. RESURF devices are described inKarmalkar, S., et al., “RESURF ALGaN/GaN HEMT for High Power Switching,”IEEE Electron Letters, Vol. 22., No. 8 (August 2001). RESURF deviceshave a p-type region formed below the channel region. The purpose of thep-doped region is to deplete the channel charge when the drain bias ishigh in order to reduce the peak field in the drift region.

The channel charge can also be depleted by a metal field-plate oroverlapping gate. The field plate is placed over the insulator betweenthe gate and the drain. The field plate 155 is illustrated in FIG. 1 andis also described in Asano, K. et al., “Novel High Power AlGaAs HFETwith a Field-Modulating Plate Operated at 35 V Drain Voltage,” IEDM, pp.59-61 (1998). The field plate 155 is placed in the drift region 160 ofthe device and is electrically connected to the gate or other voltagesource (electrical interconnection not shown in FIG. 1). However, thereis a physical gap between the gate electrode 115 and the field plate155. As discussed in Asano et al, the field plate reduces the peakelectric field near the gate edge and therefore increases the breakdownvoltage of the device. Specifically, Asano et al. reports an increase inBV_(GD) from 30 V to 47 V and attributes that increase to the presenceof the field plate.

FIG. 2 is a prior art MESFET 200 in which the field plate is configuredas an overlapping gate 255. Such a device is described in Chen,Chang-Lee et al. “High-Breakdown Voltage MESFET with a lowtemperature-grown GaAs Passivation and Overlapping Gate Structure,” IEEEEDL, Vol. 13 (No. 6) (1992). Chen, Chang-Lee et al. reports an increasein both BV_(GD) (from 25 to 35) and BV_(DS) (from 22 to 37) andattributes the increase to the presence of the overlapping gate in theMESFET. The device in FIG. 2 is otherwise identical to the device inFIG. 1 (with one other exception—the device in FIG. 2 does not have abarrier layer).

However, even with the increases in BV reported above, the reported BVfor the devices with field plates/over-lapping gates is still far belowthe BV of silicon-based laterally diffused MOS (LDMOS) devices (i.e. BVof up to 80 volts).

Other alternatives to the above devices have been proposed. A MISFETdevice 300 is illustrated in FIG. 3. MISFET 300 is very similar toMESFET 100 (FIG. 1) except for the fact that the gate 315 in MISFET 300is formed on an insulating barrier layer 350. This barrier layer is alsoknown as a gate insulating layer. Also, MISFET 300 does not have a fieldplate or overlapping gate.

The barrier layer 350 of MISFET 300 is a low temperature grown (LTG)oxide. However, because the oxide is formed from a III-V material (GaAs)the oxide has too many interface states under the gate 315. Theseinterface states cause undesirable trap-induced effects.

An alternative to the LTG oxide depicted in FIG. 3 was proposed in Ye,P., et al., “GaAs MOSFET With Oxide Gate Dielectric Grown by AtomicLayer Deposition,” IEEE EDL, Vol. 24, No. 4, p. 209-211 (2003). Theinsulator material described in this reference was aluminum oxide(Al₂O₃) and it was formed on a GaAs substrate. Ye et al. report abreakdown field as high as 9 MV/cm for the device depicted in FIG. 3.The higher breakdown field is due to the fact that the bandgap energyfor Al₂O₃ is 8 eV compared to 1.4 eV for the LTG GaAs. The insulatormaterial described in this reference had a lower interface state densitythan the previously described III-V gate oxides.

However, merely inserting a gate oxide into the device, even if thatdevice is the above-described Al₂O₃ formed by ALD, is not a globalsolution to increasing the BV for III-V devices. This is because thereare multiple sources of BV for these devices.

Accordingly III-V devices with improved resistance to BV are stillsought.

SUMMARY OF THE INVENTION

The present invention is directed to a III-V semiconductor FET that hasimproved BV_(DS) and BV_(GD). As such the devices are particularlysuited for high voltage applications. For purposes of the presentinvention, high voltage applications are those applications where thedevice is required to handle 20 volts or more in a regular operatingenvironment. In typical high voltage applications, the devices areregularly required to handle 50 volts or more.

The device of the present invention is configured to provideadvantageous electrostatic control of the electrical potential of thedevice in the drift region. For purposes of the present invention, thedrift region is the region of the device between the drain edge of thegate and the drain of the device. The design of the drift region isbalanced to achieve the dual objectives of high BV_(DS) and devicereliability versus high performance. A longer drift region provides ahigher BV_(DS) and better reliability, but lengthening the drift regionadversely affects device performance. The present invention identifies amethodology for determining the appropriate design of the drift regionto achieve this dual objective.

Basically, the invention identifies certain device features that providethe above-described electrostatic control. In one embodiment, theinvention is a GaAs passivation layer formed by ALD (atomic layerdeposition) in combination with either an overlapping gate or a fieldplate. A field plate is on the underlying passivation layer but not onor otherwise in contact with the device gate. An overlapping gate is onand in contact with the gate but extends beyond the gate and is on andin contact with the underlying passivation layer.

In the context of the present invention, the field plate or overlappinggate must have a length L_(OG) over the passivation layer and athickness of the passivation layer (h_(OG)) that are selected to providethe desired BV_(DS) for a given operating voltage. This is accomplishedby first identifying a peak field of the device that, for a givenV_(DS), will provide the desired BV_(DS) at the selected V_(DS). Forexample, when V_(DS) is 80 volts or less, a peak field of about 400kV/cm will provide a III-V device with the desired BV_(DS).

In a second embodiment of the present invention, a p-type pocket isprovided in a crystalline GaAs layer formed between the bulk substrateand the channel layer. This pocket extends laterally beneath the driftregion from about the gate to some point between the gate and the drain.Since the p-type pocket is not a layer but is laterally confined, thep-type pocket must be introduced through known techniques of maskeddiffusion or ion implantation. The mask is lithographically defined. Ifthe device has the pocket, the device is not required to have the ALDpassivation layer, although it is advantageous if the device has thepassivation layer formed by ALD in addition to the pocket. The placementof the p-type pocket is again governed by the desire to lower the peakfields in the device. In this embodiment, it is also advantageous if anoverlapping gate or field plate is provided to maintain the peak fieldof the device at a level that will provide a device with the desiredBV_(DS) (e.g. about 400 kV/cm or less during normal device operatingconditions when V_(DS) is less than about 80V). Hence, in thisembodiment, the device can be either a MOSFET (oxide interposed betweengate metal and semiconductor) or a MESFET (gate metal directly incontact with semiconductor). For purposes of placement and dimension, itis advantageous if the p-type pocket is formed no deeper than 0.5 μmbelow the channel of the III-V FET. Furthermore, it is advantageous ifthe pocket extends no further than one-third of the distance between thegate and the drain.

In either embodiment, advantageous additional features include a holesink. In the context of the present invention, this hole sink isprovided in the form of a p+ substrate or a p+ region formed adjacent tothe source region of the device (typically referred to as a p⁺ sinker).

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference numerals denote similar elementsthroughout the views:

FIG. 1 is a schematic view of a prior art MESFET device;

FIG. 2 is a schematic view of a prior art overlap MESFET/MISFET device;

FIG. 3 is a schematic view of a prior art GaAs MISFET.

FIG. 4A is a schematic of a III-V MESFET device according to oneembodiment of the present invention.

FIG. 4B is a schematic of a III-V MOSFET device according to anotherembodiment of the present invention.

FIG. 4C is a schematic of a III-V MESFET device according to a thirdembodiment of the present invention.

FIG. 5 is an I-V curve for a simulated device formed on asemi-insulating substrate.

FIG. 6 is an I-V curve for a simulated device formed on a p⁺ III-Vsubstrate.

FIG. 7 is a schematic of a III-V MESFET device of the present inventionaccording to a fourth embodiment.

FIG. 8 is a flow chart describing the method of the present invention bywhich a III-V device that is suitable for high voltage applications isdesigned.

DETAILED DESCRIPTION

The present invention is directed to a III-V device specificallyconfigured for use in high voltage (i.e. 20 volts or higher)applications and a method for determining its configuration.Specifically, the particulars of the high voltage environment (e.g.V_(DS) and BV_(DS)) are first identified. A peak electric field is thenidentified that will provide a device that will satisfy the breakdownvoltage requirement and provide a device with the requisite reliability.The device is then configured to provide a peak electric field at theidentified value of V_(DS).

In order to illustrate this methodology a sample V_(DS) and a targetBV_(DS) (off) for that V_(DS) are first selected. For purposes ofillustration a BV_(DS) (off) of about 80V is selected and the peakelectric field in the drift region is then examined in the off-state fora V_(DS) of 80 volts. These values were selected to demonstrate that aIII-V device with a breakdown voltage equivalent to that found insilicon-based devices can be obtained. Certain device features are thenconfigured such that the peak electrostatic field in the drift region is400 kV/cm and preferably less when V_(DS)=80V.

The present invention identifies a combination of features that providea III-V device for high power applications with an unprecedented BV byproviding devices with superior electrostatic control of the driftregion potential. The most important performance factors for RF powerFETs are R_(SD)(on), the maximum power gain frequency (f_(max)), theoutput conductance (G_(SD)=∂ I_(D)/∂ V_(DS)) and breakdown voltage (BV).To extract high power efficiently from power FETs at high frequency(about 1 GHz or greater) the FET must have a low R_(SD)(on), a highf_(max), a low G_(SD) and a high breakdown voltage (BV).

Two breakdown voltages are important for this analysis. The first is thedrain to source breakdown voltage (BV_(DS)) which is the largest biasthe device can sustain between source and drain with an acceptableoutput conductance. The second is BV_(GD) which is the largest bias thedevice can sustain between gate and drain with a low gate current.

Drain to source breakdown generally occurs as a result of the impactionization of electrons accelerated between the source and the drain.Impact ionization becomes large when electric fields in the FET channelbecome large. High values of V_(DS) generate high electric fieldscausing impact ionization and eventually device breakdown. Gate to drainbreakdown is also the result of high fields from the gate to the drainwhen V_(GD) is high. It results both from impact ionization between thegate and drain and from large tunnel currents from the gate to thechannel. Sometimes, a breakdown event is reversible and sometimes it isdestructive. In either situation, gate and drain biases must be reducedbelow BV to ensure a safe operating margin.

The present invention defines certain parameter ranges that provideadvantageous electrostatic control of the electrical potential in thedevice drift region. In one embodiment, described with reference to FIG.4A, the device is a III-V MESFET 400 having a source 420 and sourcecontact 405, drain 425 and drain contact 410 and gate 415 formed onchannel layer 430. The channel layer 430 in this example is n-type GaAs.In alternate embodiments, the channel layer 430 is a composite layer inwhich a semiconductor barrier layer is formed above the n-type channel.The source 420 and drain 425 are n⁺GaAs and are optional. The source 405and drain 425 contacts are not optional and contact either the sourceand drain, respectively, if present, or directly contact the channel430. The channel layer 430 is over an epitaxial layer 440 of p⁻GaAs,which in turn is formed on the p⁺ GaAs substrate 445. Channel layer 430is, typically, also formed epitaxially. A backside substrate contact 460is provided to directly contact the p⁺ substrate. A p-pocket 441 isformed in the epitaxial layer 440. The level of dopant concentration inthe p⁻layer is at least 2e¹⁵ cm⁻³.

MESFET 400 also has an overlapping gate 455 formed over and inelectrical contact with the gate 415. The overlapping gate is alsoformed on a passivation layer 450. The passivation layer 450 isinterposed between the gate 415 and the source 420 and the gate 415 andthe drain 425. The thickness 451 of the passivation layer is designatedas h_(OG) and is also the measure of the distance that separates theoverlapping gate 455 from the channel 430. The distance 456 that theoverlapping gate 455 extends from the gate 415 edge into the driftregion 416 is designated L_(OG). The distance 442 that the p-pocket 441extends from the gate into the drift region 416 is designated L_(POCK)and the depth 443 of the p-pocket 441 in the epitaxial GaAs layer 440 isdesignated d_(POCK.) The p⁻type pocket is formed by further implant of adopant species in the GaAs epitaxial layer. The dopant is again a p-typedopant species and the total dopant dose (concentration multiplied bythe thickness of the pocket) is at least 10 ₁₂ cm⁻². It is advantageousif the total dopant dose in the p-pocket is comparable in magnitude,(but not in dopant type) to the total dopant dose in the drift region.

The values of L_(OG) and h_(OG) are selected so that the peak field inthe drift region is less than about 400 kV/cm when V_(DS)=80V. Thevalues of L_(OG) and h_(OG) depend upon the characteristics of aparticular device. In one illustrative example, applicants chose tosimulate the performance of devices depicted in FIGS. 4A and 4B. Thedevice in FIG. 4B is identical to the device in FIG. 4A, except that agate insulating layer 550 is interposed between the gate 515 and thechannel 530. The passivation layer 552 lies above the gate insulatinglayer 550 but does not extend under the gate 515. The passivation layer552 may be the same material as the gate insulating layer. In deviceswithout an overlapping gate or a field plate the passivation layer isoptional. Thus, FIG. 4B is the MOSFET alternative to the. MESFETdepicted in FIG. 4A. To the extent device elements are repeated fromFIG. 4A, those elements are given a corresponding 500-series number(e.g. source 405 in FIG. 4A is 505 in FIG. 4B.)

Although applicants do not wish to be held to a particular theory,applicants believe that the difference in breakdown voltage is due inpart to the fact that, in the silicon-based LDMOS devices, there is aninsulator between the gate and the channel. By contrast, the overlappinggate or field plate MESFETs have a Schottky barrier between the gate andthe channel. Insulators have a higher breakdown field than the Schottkybarrier formed between the III-V channel and the Schottky metal (i.e.the field plate or overlapping gate). If the dominant breakdownmechanism for III-V high power devices is gate breakdown, inclusion of agood gate insulator is a possible solution.

The performance of the devices depicted in FIGS. 4A and 4B weresimulated using a standard device simulation package. Both MOSFET andMESFET structures were simulated. For the MOSFET structure, the gateinsulator layer was Al₂O₃ having a thickness of 160 Angstroms. Forpurposes of the simulations, the passivation layer was either an Al₂O₃material or an SiO₂ layer. The Al₂O₃ material was modeled to have beendeposited by ALD. That is, the surface states of the passivation layerswere presumed to be the surface states that would result if thepassivation layers were formed by ALD. For purposes of the simulation,the gate length (L_(g)) was selected as 1 μm and the drift region wasselected to be 3 μm.

In order to determine the effects of the values of L_(POCK), d_(POCK),L_(OG) and h_(OG) on the gate field and peak electric field in the driftregion, these parameters were varied for the various simulationssummarized in the following Table 1. The gate and peak fields werecomputed when the device was in the off-state and V_(DS)=80V. In thesimulations for which a particular parameter is indicated as NA, thevalue of that particular parameter was not included in the simulation.This was done to isolate the effect of the particular parameter underinvestigation on the electrical fields (and in particular the peakelectric field) of the device. TABLE 1 Simulation Results Gate PeakPassivation H_(OG) L_(POCK) d_(POCK) Field Field Device Type LayerL_(OG (um)) (um) (um) (um) kV/cm kV/cm Set 1: A ALD Al₂O₃ NA NA 0.75 0.2386 386 MOSFET B ALD Al₂O₃ NA NA 0.75 0.4 525 525 C ALD Al₂O₃ NA NA 0.751.0 735 735 D ALD Al₂O₃ NA NA 1.5  0.2 880 880 Set 2: A ALD Al₂O₃ 0.750.2 NA NA 430 430 MOSFET B ALD Al₂O₃ 0.75 1.2 NA NA 610 610 C SiO₂ 0.750.2 NA NA 623 623 D ALD Al₂O₃ 1.5  0.2 NA NA 607 655 Set 3: A ALD Al₂O₃NA NA 0.75 0.2 280 380 MESFET B ALD Al₂O₃ NA NA 0.75 0.4 637 637 C ALDAl₂O₃ NA NA 0.75 1.0 872 872 Set 4: A ALD Al₂O₃ 0.75 0.2 NA NA 545 545MESFET B ALD Al₂O₃ 0.75 1.2 NA NA 770 770 C SiO₂ 0.75 0.2 NA NA 830 830For the simulations in which d_(POCK) was varied and L_(POCK) was heldconstant, the results demonstrate that, the closer d_(POCK) is to thechannel, the lower the gate field and the peak electric field are in thedrift region. This demonstrates that the pocket works to deplete thechannel charge. Note that an increase in the value of L_(POCK) alsocauses the electric fields to increase significantly. This is seen bycomparing Set 1 devices A and D in which L_(POCK) has been increasedfrom 0.75 to 1.5 μm. When L_(POCK) is increased to 1.5 μm, the fieldsbecome unacceptably large. So much so that further reduction in thedepth of dpocK no longer adequately compensates for the increase and thedesired peak field (i.e. below 400 kV/cm) cannot be achieved.

These results demonstrate that d_(DOCK) has a demonstrable effect on theelectric fields in the device region. Thus the value of d_(DOCK) isselected to provide the peak electric field in the drift regionspecified herein. In this regard, the pocket can actually extend intothe channel region as long as the channel current is not significantlyreduced. One skilled in the art can determine the necessary trade-offbetween the benefit of having the pocket extend into the channel regionto control electric fields in the drift region and any negative impacton channel current as a result of this configuration. Generally, thehigher the dopant concentration in the channel, the closer the pocketshould be to the channel. Also, for purposes of the impact on theelectric field in the drift region, there is no advantage to having thep-pocket extend under the gate beyond the drain side of the gate. Thatis, the p-pocket need only extend into the drift region from the drainside of the gate and not from some point closer to the source. Thus,although the devices in FIGS. 4A and 4B illustrate the pocket asextending somewhat under the gate, this is not required, nor evenadvantageous (beyond ensuring that the p-pocket is indeed positionedunder the drain-side edge of the gate).

Based upon the foregoing, it is advantageous if the d_(POCK) is lessthan 0.5 μm and L_(POCK) is no more than ⅓ the distance of the driftregion 416. This is true for either embodiment depicted in FIGS. 4A and4B. Furthermore, it is advantageous if the total dopant dose in thepocket be at least about 10¹²/cm². The peak doping concentration of thep-pocket should be at least 5×10¹⁶ cm⁻³. Again it is advantageous if thetotal dopant dose in the p-pocket is comparable in magnitude (but not indopant type) to the total dopant dose in the drift region. Since theepitaxial layer 440 in which the p-pocket is formed is a III-V material,it is advantageous if the p-pocket dopant is beryllium (Be). One skilledin the art is well aware of numerous techniques that can be used tointroduce such dopants into such materials. Consequently implantationtechniques are not described herein.

The effect of the values of L_(OG) and h_(OG) on the electric fields inthe drift region were also evaluated. Again, simulations were made forthe MESFET device depicted in FIG. 4A and the MOSFET depicted in FIG.4B. Table 1 describes the field magnitudes in MOSFETs in which thepassivation layer was Al₂O₃. For two simulations L_(OG) was 0.75 μm.When h_(OG) was increased from 0.2 μm to 1.2 μm the peak field wasobserved to increase dramatically. In another simulation, the Al₂O₃ wasreplaced with SiO₂ and the h_(OG) was 0.2 μm.

Again, a significant increase in peak field was observed as a result ofthis change. This observed increase is attributed to the fact that Al₂O₃has a much higher relative dielectric constant (K=9) than SiO₂ (K=3).The critical parameter is the “electrical thickness” of the passivationfilm. Herein, the electrical thickness is defined to be the equivalentthickness with respect to the dielectric constant of Al₂O₃.Specifically, the electrical thickness (E_(T)) is calculated relative toAl₂O₃. That is, the ratio of the dielectric constant of Al₂O₃ to thedielectric constant of the passivation film, times the thickness of thefilm, yields E(_(T)). This is also expressed as K(_(A1203))h_(OG)/K_(f), or 9 (H_(OG)/K_(f)) where K_(f) is the average dielectricconstant of the passivation film. Consequently, even when the value ofh_(OG) is small, the smaller K of SiO₂ has the effect of increasing theelectrical thickness of h_(OG). The passivation layer may be composed ofmultiple layers of different materials having different values of K. Insuch a case, K for the passivation layer is understood by those skilledin the art to be the effective dielectric constant of the layer. Inparticular, it is advantageous if E_(T) is less than about 0.3 μm.

In yet another MOSFET simulation, L_(OG) was increased to 1.5 μm. Theeffect was observed to be analogous to the observed effect when L_(POCK)was increased. It is clear that for an OG MOSFET, h_(OG) must be thickerthan the gate oxide thickness. From the foregoing, it is advantageousif, for the embodiments of the present invention that possess anoverlapping gate 455 (FIG. 4A), 555 (FIG. 4B), that the overlapping gateextend no further than about one-third of the distance into the driftregion (e.g. 416 in FIG. 4A). It is also advantageous if the value ofhoc is such that the h_(OG) divided by K for the passivation layer isabout 0.3 μm or less. Preferably h_(OG)/K is 0.1 μm or less. In thisregard, it is advantageous if the passivation layer is made of amaterial having a dielectric constant that is greater than about 5. Itis also advantageous if the passivation layer is deposited by ALD toreduce the interface state (i.e. trap) density associated with prior artpassivation layers such as those described in Chen et al.

Table 1 further describes simulations similar to those described abovebut for the MESFET depicted in FIG. 4A. That is, the effect ofvariations in L_(OG), h_(OG) and passivation layer on the electricfields in the drift region were simulated and the results are reportedin Table 1. For the MESFET, as for the MOSFET, increases in peakelectric field were observed with an increase in h_(OG) and when thehigher K Al₂O₃ passivation layer was replaced with the lower K SiO₂passivation layer. All other things being equal, the MESFET embodimenthas a higher peak field than an otherwise identical MOSFET (thebreakdown field of the MOSFET oxide is higher than the Schottkey gatebreakdown field in the MESFET).

In another embodiment, the overlapping gate 455 depicted in FIG. 4A isreplaced by a field plate structure 656 depicted in FIG. 4C. Again, tothe extent a device element is repeated from FIG. 4A it is given acorresponding 600-series number in FIG. 4C (e.g. source 405 in 4A is 605in 4C). FIG. 4C depicts the field plate in a MESFET structure. In analternate embodiment (not shown) the field plate is incorporated into aMOSFET structure. In this embodiment it is again advantageous if thepassivation layer 650 is an ALD deposited oxide to reduce the interfacestate density. It is advantageous if the oxide is an oxide with a high Ksuch as Al₂O₃. One advantage of the field plate structure is that, whengrounded to the device source, the drain capacitance is screened fromthe gate.

Although most of the simulation results reported in Table 1 have a peakbreakdown field higher than the desired peak field of 400 kV/cm, Table 1illustrates the methodology whereby one skilled in the art can obtain aIII-V device with a peak breakdown field, and, more importantly, a BVthat will satisfy the requirements for high voltage applications.Referring to the flow chart in FIG. 8, the first step 800 is to identifythe maximum operating voltage (V_(DS))of the device. In high voltageapplications, this will be greater than 20 V, and typically in excess of50 V. Based upon the operating voltage, a BV_(DS) of the device is alsoselected. Typically BV_(DS) is higher than V_(DS). In step 810, a peakfield for the device is selected based upon the desired BV_(DS). In step820, dimensions for the channel and drift region are selected. Aspreviously noted, drift region dimensions are selected by balancing thebenefit of a longer drift region vis-à-vis BV_(DS) with the adverseeffects that a longer drift region has on device performance (higherI_(ON) and longer device response time). The doping in the channel isalso selected to provide the device with the desire I_(on). In highvoltage applications, I_(on) is advantageously greater than about 100μA/μm. In step 830, device features are selected to provide a devicewith the desired peak field. The features selected are either an ALDbarrier layer/gate dielectric layer in combination with either a fieldplate of a p-pocket. In the case of a p-pocket, it can be employedindependently from the ALD barrier layer to achieve the objectives ofthe present invention.

Although the embodiments described thus far have been depicted as formedon a p⁺ substrate, with an electrical connection between the source andthe substrate, the present invention contemplates semi-insulatingsubstrates as well. In the present invention the same approach can beused to control the peak fields for devices formed on p⁺ andsemi-insulating substrates.

Although the substrate type has little effect on the breakdown voltageof devices in the off-state it can have a large effect on thecharacteristics of the device in the on-state. When electric fields inthe drift region are large and there is an electron current flowing fromthe source to the drain, holes are generated by impact ionization. Theseholes must flow out of the device through a low voltage electrode. In aMOSFET formed on a SI substrate, the holes must exit the device throughthe source contact. In a MESFET on a SI substrate they can exit throughthe source and through the gate electrodes. In either case, if the holessee a large net impedance to ground, a net hole charge can build in thebulk of the device that will alter the potential under the gate andresult in an undesirable effect on the device characteristics. Inparticular, the drain current can exhibit a “kink” and a large outputconductance (G_(DS)). These holes eventually will lead to a snap-backbipolar-induced breakdown.

Simulations were performed from which the I-V curves were obtained forIII-V MOSFETs formed on silicon and p⁺ substrates. The MOSFET simulatedhere shares the properties of the device simulated for the resultsreported in Table 1, Set 1, Device A. From the results of thesimulations (using gate voltages of −1, −2, and −3 volts) I-V curveswere obtained and are illustrated in FIGS. 5 and 6.

Semi-insulating substrates cause all holes to exit through the sourceelectrode. From the I-V curve in FIG. 5, a relatively large impedancefor the holes to ground is observed from the kink effect and largeG_(DS) illustrated in FIG. 5.

The same device was simulated with a p⁺ substrate. Again, an I-V curvewas obtained from the simulations as described above. The resulting I-Vcurves illustrated in FIG. 6 are free of the kink effect and show verysmall G_(DS). Similar results were observed from simulations of MESFETstructures on SI and p⁺ substrates. In this device having a p⁺ substrateand a substrate contact, the holes can also exit the device through thesubstrate contact.

Thus, contrary to conventional wisdom which favors semi-insulating III-Vsubstrates because they reduce drain capacitance, applicants havediscovered that forming III-V devices on p⁺ substrates provides certainadvantages. Specifically, the p⁺ substrate provides an electrical sinkfor holes, which is very important for high BV(on). This is particularlytrue for MOSFETs, because, due to the fact that the MOSFET has no gatesink for holes, the hole impedance to ground is higher for a MOSFET thanfor a MESFET.

Therefore, it is a further aspect of the current invention to form III-VMOSFETs and MESFETs on p⁺ substrates.

Another embodiment of the present invention is illustrated in FIG. 7. Tothe extent that the device elements from FIG. 4C are repeated in FIG. 7,those elements are given a corresponding 700-series number. The devicedepicted in FIG. 7 is a MESFET 700 with an overlapping gate 755. A topside p-contact 721 is provided near the source to collect holes (i.e. tofunction as an electrical sink for holes). The p contact is p⁺ GaAshaving a doping concentration of at least 10¹⁷ cm⁻³. The top-sidecontact 721 is connected to the source contact 705 and thus, the sourcevoltage. In this configuration it is not required that the substrate bep⁺ to achieve the advantages of the p⁺ substrate described above. Inthis embodiment the substrate 745 is either a p⁺ substrate or asemi-insulating substrate. In fact, in this embodiment thesemi-insulating substrate is somewhat advantageous because it hassuperior drain capacitance compared to the p⁺ substrate. The p-sink canbe formed by implantation of a suitable p-type dopant such as Be.

The hole sink 721 depicted in FIG. 7 is in electrical contact with theburied p-pocket 741. It is a further aspect of the current inventionthat the hole sink be used in FETs formed on p⁺ substrates.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for fabricating a III-V device for high power applicationscomprising: determining the V_(DS) for a device formed on a substratemade of a III-V semiconductor material; selecting a desired breakdownvoltage (BV_(DS)) for the device based upon the determined value forV_(DS); selecting a peak electric field for the device based upon thedetermined value for V_(DS) and the selected value for BV_(DS);selecting a device channel length and a device drift region length;selecting device features that will provide the selected peak electricfield for the device at the determined value of V_(DS) for the device;and taking steps to complete the device.
 2. The method of claim 1wherein the device features are selected from the group consisting of apassivation layer formed by atomic layer deposition on the devicechannel and a p-type region formed in proximity to the channel layerformed thereover, wherein the proximity of the p-type region is selectedso that at least some portion of the p-type region is no more than about0.5 μm from the device channel and wherein at least a portion of thep-type region extends from a region proximate the drain side of thedevice gate into the drift region of the device.